A typical information display system forms a video image by mapping a series of positions, intensities and often color signals onto a display device. These signals can represent lines, circles, graphics, symbols or camera images. The goal of any such device is to present the information ergonomically. Perceptible artifacts such as smearing, flashing, flickering, jittering, wiggling, non-linearity and positional inaccuracy all detract from the quality of the display. The input signals to a display device are often analog electrical signals and these signals are subject to noise, drift, and other imperfections.
The most prevalent display device is the Cathode Ray Tube (CRT) and typical display systems are designed to be able to utilize CRTs. In a CRT, an electron beam is swept or moved across the display surface and the intensity is modulated to form the image. The image on a CRT does not persist indefinitely and in order maintain the image the beam must continually retrace and refresh the image. In a raster video system the position information is encoded in time and the positional accuracy is determined by the ability to synchronize the display to the image source. In a stroke video system, the position information is encoded in the amplitude of the input signals. The accuracy of a stroke video system is determined by the ability to accurately follow the amplitude signals. In both stroke and raster systems, the intensity and color of the image are often encoded in the analog amplitude of input signals.
Historically, both stroke and raster image systems were used on avionics platforms. Raster image systems were used to accept TV or camera images and stroke systems were often used for computer graphic information, because of Stroke's high resolution with low memory requirement characteristics. A single CRT was often used to display both raster and stroke input information.
Flat Panel Displays (FPD) such as Liquid Crystal Displays (LCD) have been replacing CRT displays in many applications. In particular, avionics applications have been shifting to LCDs because they use less space, weight less and are more reliable than CRTs. Often the input signals are not redesigned and updated at the same time and the FPD must accommodate the legacy input signals. A FPD typically uses a digital, discrete, pixel position addressing scheme compared to the typically smooth, analog position addressing of a CRT. The legacy signals must be converted by the FPD from their original format to a format that is useful for the new FPD.
The conversion of analog raster input signals for display on an FPD is a well-known problem. The ability to synchronize the display to the raster image source makes the pixel addressing accurate and resistant (but not immune) to noise issues. However, the same techniques cannot generally be utilized on stroke inputs. Any noise or errors on the positional inputs of stroke video can result in temporary illumination of pixels on the LCD. The temporary illuminations can cause the image to appear to be wiggling, jittering and/or flashing. This occurs because the noise typically has a random component and display inputs are trying to repeat the same image at a high rate (typically at least 50 Hz.). Each time the display input redraws the image, the position inputs are shifted randomly by the noise causing the display to appear to be changed each time the display is refreshed and redrawn.
Although stroke positional inputs can be high resolution, the ability of a CRT to display fine stroke details is typically limited by inertia of the electromagnetic field that is used to deflect and sweep CRT's electron beam. This inertia limits the accuracy of the beam's position when starting, stopping and changing directions. Thus, fine details such as characters and symbols can appear distorted from their intended appearance. Often the stroke signal generator/computer will compensate for some of these distortions in the input signal. It is not desirable for the FPD to replicate either the distortions of the CRT or signal generator.
U.S. Pat. No. 3,786,479, issued to Brown et al., describes a computer-based video display system that receives binary coded image information from a host of sources. The image information is subsequently converted into stroke, or vector, information. This vector information is then converted into raster information that is stored on a magnetic disk and later displayed on a CRT.
U.S. Pat. No. 4,458,330, issued to Imsand, et al. describes a converter that receives and stores vectors within a given region or band. The stored vectors are serially selected and converted to raster coordinates. The converter then determines if the coordinates should be output. After making this decision for all the coordinates, the output is generated.
U.S. Pat. No. 4,658,248, issued to Yu, describes a method for generating stroke characters for use in a display system. Data signals are received that identify the character type and character location. Some of the data signals are processed to identify a memory location that holds instructions on generating stroke signals for identified characters. These instructions are processed to generate stroke vectors that are subsequently connected and scaled.
U.S. Pat. No. 5,557,297, issued to Sharp et al., discloses a system for displaying calligraphic video on raster displays. This system first converts analog stroke data into a raster image. By digitizing the stroke signals' intensity to a fraction of the pixel resolution, this invented system avoids problems with high scan conversion rates and large buffers. Noise within the image is reduced by saving the first and last point on the line or by using anti-aliasing disks that limit changes in intensity to a pre-selected amount, such as 3 of the pixel intensity.
U.S. Pat. No. 5,929,865, issued to Balz et al., describes a method of sorting and converting two-dimensional graphic images raster lines. Shape data, such as a circle, defined by a two-dimensional coordinate system is received. This shape data is then decomposed into individual vectors having coordinates within the defined coordinate system based on a first criterion. The determined coordinates are later sorted by a second criterion, which is used in forming raster lines.
U.S. Pat. No. 5,396,582, issued to Kahkoska, describes a raster-to-vector conversion system. This conversion system determines if a pixel is lit. If the pixel is illuminated, this system identifies a unique vector with beginning coordinates that match the coordinates of the illuminated pixel. When the vector is identified, memory is updated and the beginning and ending coordinates of the vector are sent to the plotter.
U.S. Pat. No. 5,969,699, issued to Balram et al., describes a digital filter. This filter converts line and arc data into a raster image by repetitively matching collected data to predefined templates.
U.S. Pat. No. 6,226,400, issued to Doll, discloses defining color borders in a raster image. The patent converts a raster image into a vector image without significantly varying the image by converting the color borders of the raster image into mathematical representations.
Presently existing techniques are in general concerned with converting exactly one frame of analog data for display into a bit mapped raster formats. There is a need to reduce the effects of frame-to-frame or time varying component of the noise, which can make the image appear to flash, flicker, wiggle and/or jitter. Because the human visual system is efficient at detecting changes, processing frames independently from one another can exacerbate the effects of noise. For example many techniques attempt to anti-alias lines and vectors. This is a form of smoothing and on a single frame of data it can improve the appearance. However, if the line or vector is being drawn wider, any noise from frame to frame is spread over a larger area and the eye can more easily detect variations over time in the expanded area. Another example of making the noise worse can occur whenever a conversion algorithm chooses a starting pixel on the FPD (or in the frame buffer) that corresponds to the start of a stroke line or segment and then processing from that point. The problem here is that frame-by-frame there is no guarantee that the start pixel will be the same. The algorithm will have time varying artifacts across the entire length of the line or curve, again more easily detectable than if the change had occurred on a single pixel.
Despite the developments in the area of display systems, conventional solutions do not always effectively eliminate time varying transients when displaying an analog signal on a discrete pixel element basis, such as an LCD. In a conventional stroke conversion solution there is a need for improving the translation and accurately positioning of highly detailed features, such as symbols onto a FPD. Thus, a need still exists for a conversion system that reduces time varying noises and artifacts that can distract or misinform the user.